1. Technical Field
The present application relates to generators, and more particularly to a generator mounted on a vehicle such as passenger cars and trucks.
2. Description of the Related Art
The generator mounted on a vehicle (i.e., on-vehicle generator) supplies power to a battery to be charged or supplies necessary power to various electrical loads via a charge line which is connected to the output terminal of the on-vehicle generator. While the generator is generating power, when the output terminal or the battery terminal is disconnected unexpectedly, transient high voltage spikes i.e., load dump occurs. The load dump voltage may exceed 100 volts depending on load current. Since such high voltage spikes may cause damage to the electrical load or damage of devices used in the generator, a countermeasure is required to prevent this phenomenon.
In this regard, Japanese Patent Application Laid-Open Publication No. 1997-219938 discloses an on-vehicle generator in which occurrence of high voltage spikes is suppressed by a protecting operation. The protecting operation is such that low-side devices of a bridge circuit of the generator are configured by MOS transistors and the MOS transistors are controlled to be turned ON when the output voltage of the generator exceeds a predetermined reference voltage due to occurrence of the load dump, thereby suppressing occurrence of the high voltage spikes.
In the on-vehicle generator disclosed in the above-described patent document, when the respective MOS transistors as low-side devices (low-side MOS transistors) of the bridge circuit are controlled to be turned ON whereby the output voltage decreases to be within the predetermined reference voltage, the respective MOS transistors turn OFF again. Therefore, normal rectification by the bridge circuit is resumed.
In the on-vehicle generator disclosed in the above-described patent document, the low-side MOS transistors turn ON and OFF at a switching rate faster than the period of rectification when the phase voltage exceeds the predetermined reference voltage. As a result, it needs a longer time to consume the charge energy of the gate pulse voltage so that occurrence of the high voltage spike cannot be terminated immediately.
For terminating the high voltage spike in a short period of time, it is considered that the low-side MOS transistors can be kept in an ON state when the phase voltage of the on-vehicle generator exceeds the predetermined reference voltage. However, in some of the on-vehicle power generation control apparatus (regulators), control operation starts when detecting start of power generation triggered by a switching frequency corresponding to the phase voltage exceeding a predetermined threshold voltage, and alerts to notify occurrence of abnormality when starting of the power generation cannot be detected.
Therefore, if the period for the low-side MOS transistor being OFF becomes longer, the phase voltage continues to be lower than the predetermined threshold voltage even if the on-vehicle generator is rotating. Hence, a stop-condition of the power generation may be erroneously detected.
In this case, assuming the on-vehicle generator supplies initial excitation current to the field windings in order to start the power generation, redundant current would flow through the field windings and phase current flowing through the low-side MOS transistor would increase. Considering the efficiency of the power generation and the heat dissipation, it is not preferable to control the on-vehicle generator in such a way. Moreover, when a stop-condition of the power generation is erroneously detected so that the on-vehicle generator mistakenly to performs an alert operation, the driver of the vehicle will be confused and wrong signals may be transmitted to an external devices.